Spaceflight & Fuel

Gravity

We use the term gravity everyday to describe the force pulling us downward, whether on a planet or a station. However it is essential to spaceflight to remember that all things with mass exert gravity and are effecting by other's gravity. A star will pull on an orbiting planet, but that planet also pulls on the star. As objects move through space, their gravitational attraction causes extraordinary, but thankfully predictable behavior which we can use to plan our voyages.

Orbit

When an object under a certain velocity and mass approaches another object with a much greater mass, such as that of a planet, gravity causes it to travel in a circle around that massive object. This behavior is called an Orbit. Objects may orbit a shared object, such as multiple planets orbiting the same star. Objects may also orbit objects that are orbiting other, even larger objects, such as a moon orbiting a planet orbiting a star.

Orbits may be adjusted in a number of ways. Burning in the direction of a craft's velocity will widen the orbit and eventually reach escape velocity, at which point it breaks orbit. Burning away from the craft's velocity will tighten the orbit and eventually intersect with the surface. Burning up or down relative to the orbital plane will cause the orbit to incline or decline.

Orbits are best altered when burns occur either at the closest point to the celestial body, or at the farthest point.

Encounter

Objects in motion through space will generally stay in motion until a force causes a change in their velocity. One such force is the gravitation attraction of a celestial body. When an object approaches another object close enough that it exerts a gravitation force on it, this is called an encounter. Depending on the mass and proximity of the object, this could change course only slightly, however it could also cause an extreme change in the craft velocity to the point that it establishes a new and unintentional orbit. This can be used intentionally to achieve a desired velocity while conserving fuel, and is often called a slingshot or gravity-assist.

Note that passing close to a non-gravitationally significant objects, such as other spacecraft, is called an intercept instead.

Intercept

To travel to an object which does not exert a significant gravitational force, one must plot an intercept course. This course predicts the object's trajectory and speed and then plots its own trajectory and speed such that the two objects travel will meet in the same point and time.

Since neither object has a gravity well, if neither craft changes course, they will either collide, or pass by. Therefor, if the mission of an intercept course is to dock with its target or engage in combat, then it will need to burn to match its velocity before the intercept occurs.

Escape Velocity

Since most objects have already found an orbit around their nearest celestial body, and these orbits generally do not change, in order to travel to other destinations, a craft must break an orbit. To break orbit, an object must reach a certain velocity which exceeds the gravitational pull. At this point, the object breaks free, making it possible to encounter other celestial bodies and travel throughout the solar system. It is important to note that if an object breaks free of its local orbit, it may still be under the influence of other more massive objects such as a star.

Burn

Contrary to ancient media interpretations of spacecraft, the majority of space travel is spent with the engines off. Once a spacecraft reaches its desired velocity, it no longer needs to use its engines aside from generating power. When engines are activated, this is called a burn. Burns cause a change in a spacecraft's velocity at the expense of engine fuel. Changes in velocity may be for a number of purposes including accelerating toward a destination, or slowing down on approach to an object. Maneuvering thrusters are specialized engines designed to help a craft make other more precise burns.

Landing

To navigate a craft to the surface of a celestial body, it must decrease its velocity until its orbit intersects the surface. It is important to note that actually landing without damage on the surface requires that the object have nearly zero velocity when it lands. This can be done partially be allowing air resistance to slow the craft down, however terminal velocity must be further negated with the use of hoglium propulsion, engine burns, or maneuvering thrusters.

Taking Off

To navigate a landed craft into space, it must accelerate away from the surface until either establishes a circular orbit, or exceeds escape velocity to the point that it breaks free of the gravitational influence.

Docking

Objects that do not exert a gravitation force are much easier to "land" on. Examples of these objects are carriers, fuel tankers, or space stations. Because they do not cause objects to orbit them, much less fuel is needed to safely decelerate. However, since there is no gravity well to "trap" a spacecraft, more precision is required in order to prevent overshooting the target. First, a spacecraft must plot a course and velocity that will cause it to meet its target. Then, upon intercepting that target, it must burn until their velocities match. From here, it may perform small maneuvers to dock with its target, or alternately launch a smaller craft to dock or land in a hangar.